sonos app cant find system

​@hrdwck, Please describe your network:

Router model, is there a Wi-Fi mesh system (model), access points (model), repeaters / powerline adapters, network switches (model), etc. Are you running Sonos on S2 or S1? Are you operating Sonos over Wi-Fi, or is at least one component hardwired to the router or a network switch?

Standard UK WiFi ISP.  Nothing hardwired.  No mesh. Standard ISP router. Sonos 2 app. Running one beam gen 1, 7 Ones, one Sub Mini. 
 

often use VPN, but problem persists if VPN is on or not. 

​@hrdwck 

Obviously the app can‘t see your system/devices on the network. So there seems to be an issue in network communication. I recommend you contact Sonos support, send a diagnostic and let them do some analyses. 
What else you can check on your side…

• have you set fixed wifi channels for 2,4 and 5 GHz? If so, try other channels… for 2,4 GHz channel 1,6 or 11 would be best choices 
• is band steering activated?
• are there any firewall rules set on your router?
• is there an active add blocker like pi-hole running?

• Try disabling band steering as a test. On ISP routers, it is often poorly implemented
• Is the router set to "auto channel"? Please set a fixed channel on the 2.4 GHz band by testing wireless channels 1, 6, and 11. Also, set the channel width to 20 MHz if 40 MHz is currently active
• VPNs and Sonos don't get along

https://support.sonos.com/article/reduce-wireless-interference

https://support.sonos.com/article/improve-your-sonos-products-wifi-connection

You can try turning your phone airplane mode on/off to reset the phone network connections. That might help reset things after using the VPN or your cell network.

​@hrdwck 

To be sure that everything regarding to network does a reset, I recommend a restart of your router, your Sonos devices and also of your smartphone running the Sonos app.